Final answer:
The outer dense shell of bone that constitutes most skeletal mass is known as compact bone, which provides support and protection under the periosteum and in the diaphyses of long bones.
Step-by-step explanation:
The outer dense shell of bone that makes up most of the skeletal mass is the compact bone. This type of bone tissue is very hard and strong and can be found under the periosteum and in the diaphyses of long bones, providing support and protection. Its functional unit is the osteon, which is organized into rings with a central Haversian canal for blood vessels, nerves, and lymphatic vessels. Spongy bone, in contrast, is found at the center of bones and is lighter and more porous than compact bone. Bone marrow is found inside the pores of spongy bone and is responsible for the production of blood cells.